linux-kernel - Re: [PATCH v9 1/9] kho: make debugfs interface optional

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Message-ID: <CA+CK2bByYPJXSNOh6R3swqFrGsS02m3Dfh=ZU7YhNjNX6siyqg@mail.gmail.com>
Date: Sat, 8 Nov 2025 13:13:11 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: "Yanjun.Zhu" <yanjun.zhu@...ux.dev>
Cc: akpm@...ux-foundation.org, brauner@...nel.org, corbet@....net, 
	graf@...zon.com, jgg@...pe.ca, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org, masahiroy@...nel.org, 
	ojeda@...nel.org, pratyush@...nel.org, rdunlap@...radead.org, rppt@...nel.org, 
	tj@...nel.org
Subject: Re: [PATCH v9 1/9] kho: make debugfs interface optional

On Fri, Nov 7, 2025 at 6:36 PM Yanjun.Zhu <yanjun.zhu@...ux.dev> wrote:
>
>
> On 11/7/25 4:02 AM, Pasha Tatashin wrote:
> > On Fri, Nov 7, 2025 at 7:00 AM Pasha Tatashin <pasha.tatashin@...een.com> wrote:
> >>> Hi, Pasha
> >>>
> >>> In our previous discussion, we talked about counting the number of times
> >>> the kernel is rebooted via kexec. At that time, you suggested adding a
> >>> variable in debugfs to keep track of this count.
> >>> However, since debugfs is now optional, where would be an appropriate
> >>> place to store this variable?
> >> It is an optional config and can still be enabled if the live update
> >> reboot number value needs to be accessed through debugfs. However,
> >> given that debugfs does not guarantee a stable interface, tooling
> >> should not be built to require these interfaces.
> >>
> >> In the WIP LUO [1] I have, I pr_info() the live update number during
> >> boot and also store it in the incoming LUO FDT tree, which can also be
> >> accessed through this optional debugfs interface.
> >>
> >> The pr_info message appears like this during boot:
> >> [    0.000000] luo: Retrieved live update data, liveupdate number: 17
> >>
> >> Pasha
> > Forgot to add link to WIP LUOv5:
> > [1] https://github.com/soleen/linux/tree/luo/v5rc04
>
>
> Thanks a lot. I’ve carefully read this commit:
> https://github.com/soleen/linux/commit/60205b9a95c319dc9965f119303a1d83f0ff08fa.
>
> To be honest, I’d like to run some tests with who/luo, including the
> selftest for kho/luo. Could you please share the steps with me?
>
> If the testing steps have already been documented somewhere, could you
> please share the link?

Currently the test performs in-kernel tests for FLB data, it creates a
number of FLB for every registered LUO file-handler, which at the
moment is only memfd.

It works together with any of the kexec based live update tests. In
v5, I introduce two tests:
luo_kexec_simple
luo_multi_session

For example, with luo_multi_session:
# ./luo_multi_session
# [STAGE 1] Starting pre-kexec setup for multi-session test...
# [STAGE 1] Creating state file for next stage (2)...
# [STAGE 1] Creating empty sessions 'multi-test-empty-1' and
'multi-test-empty-2'...
# [STAGE 1] Creating session 'multi-test-files-1' with one memfd...
# [STAGE 1] Creating session 'multi-test-files-2' with two memfds...
# [STAGE 1] Executing kexec...

... reboot ...
After reboot:

$ ./luo_multi_session
# [STAGE 2] Starting post-kexec verification...
# [STAGE 2] Retrieving all sessions...
# [STAGE 2] Verifying contents of session 'multi-test-files-1'...
# [STAGE 2] Verifying contents of session 'multi-test-files-2'...
# [STAGE 2] Test data verified successfully.
# [STAGE 2] Finalizing all test sessions...
# [STAGE 2] Finalizing state session...
#
--- MULTI-SESSION KEXEC TEST PASSED ---

Dmesg data, shows that in-kernel live update test was also successful:

[    0.000000] luo: Retrieved live update data, liveupdate number: 1
[    0.034513] liveupdate test: test-flb-v0: found flb data from the
previous boot
[    0.034517] liveupdate test: test-flb-v1: found flb data from the
previous boot
[    0.034518] liveupdate test: test-flb-v2: found flb data from the
previous boot
[    0.676891] liveupdate test: Registered 3 FLBs with file handler: [memfd-v1]

Pasha

> Best Regards,
>
> Yanjun.Zhu
>